Metarhizium anisopliae

Today, we are featuring the insect-killing fungus Metarhizium anisopliae. I have previously written about a related species that specialises on orthopterans (grasshoppers, locusts) and all species in the Metarhizium genus are dyed-in-the-wool insect killers - some of them are used as biological insecticides. There is even ongoing research looking into ways of loading them with scorpion venom to fight mosquitoes which spread malaria

Metarhizium anisopilae growth from termite cadaver
Image from Fig. 1 of the paper

Metarhizium anisopliae infects a variety of insects and in the study we are featuring today, the host they were presented with were termites. But M. anisopliae is not alone in their taste for these blind social insects. Termites can also fall victim to Aspergillus nomius - a fungus that usually lives as a saprophyte (feeding off dead things), but can sometimes be a parasite when the opportunity arises. Aspergillus nomius can grow very well by feasting on dead termites, but it has one problem; being an opportunistic "sometime" parasite, it is not very good at actually killing termites - in fact it is very bad at it.

When healthy termites are exposed to the spores of A. nomius, they are unaffected. Termites only succumb when exposed to an extremely high dose of spores (five million spores per gram of sand in the enclosure the termites were housed in) and even then, after more than 10 days, only a tenth of the exposed population died. However, when exposed to M. anisopliae at a much lower dose (five hundred thousand spores per gram of sand), the termites died in droves, as expected. When the termite population was exposed to a fifth of the dose of M. anisophliae as had been tested with A. nomius (one million spores per gram of sand), the entire experimental population was wiped out after a week

Aspergillus nomius growth from termite cadaver
Image from Fig. 1 of the paper

In additional experiments where termites were exposed simultaneously to equal doses of spores from both fungi, they died at the same rate as those exposed to the equivalent dose of M. anisopilae sans A. nomius, showing that the M. anisopliae was the true killer and A. nomius did not contribute to bringing down the termites. But despite its role in mixed infection, the dedicated parasite M. anisopliae did not get to reap all the reward for its work in mixed company. Instead, it is out-competed by the opportunistic A. nomius, with termites cadaver killed by mixed infections sprouting more A. nomius.

This study illustrates the context-dependency nature of harm and competition. Ecological competition between parasites often involves trade-offs in a number of traits, and traits that allow a parasite to successfully overcome a host's defences do not necessarily makes it a good competitor when confronted with other parasites. In this particular case, the usually saprophytic A. nomius can't take down a healthy termites on its own, but given the chance through a true killer M. anisopliae, it'll step in and take over completely.

Reference:
Chouvenc, T., Efstathion, C.A., Elliott, M.L., Su, NY. (2012) Resource competition between two fungal parasites in subterranean termites. Naturwissenschaften 99: 949-958

Encarsia inaron

On this blog, we have covered many stories of either parasite cleverly evading the host's defences or the host valiantly fighting back against these bodily invaders. But sometimes, both parties lose out on this fight, and today we are looking at such a case.

Photo by Mike Rose (source: Natural History Museum)

Encarsia inaron is a tiny parasitic wasp no longer than 0.5 mm in length. It was introduced into North America in 1989 from Europe to control the ash whitefly (Siphoninus phillyreae) a sap-sucking insect which itself hails from Europe and the Mediterranean, and has become an established pest in North America and elsewhere in the world. Encarsia inaron lays its eggs in the nymphal (immature) stages of whiteflies. Like most parasitoids, the wasp larvae use the host's body as an incubator and a larder until they are ready to mature into adults, at which point they kill the host by bursting out of its body.

In addition to the ash whitefly, which it was introduced to control, E. inaron also infects a number of other whiteflies (as you will see below). For long-time readers of this blog, you might remember earlier in the year we featured a parasitic wasp that infects aphids and why picking the right-sized host is very important for the survival of its offspring. This also applies to E. inaron but in a different way. If the wasp infects a whitefly nymph that is too far along in its development, then the host would reach adulthood before the wasp larva can complete its development. And unlike other parasitoid wasps, E. incaron is incapable of delaying its host's developmental schedule.

Once the whitefly becomes an adult, rarely will the wasp ever emerge as an adult. While it may seem that in this case the whitefly has won simply by reaching maturity before its parasitoid, that is not exactly the case. Instead, it is a pyrrhic victory - the adult whitefly is still carrying the wasp larva inside it and this burden reduces the number of eggs that it can produce by more than half and significantly shortens its lifespan.

Considering the cost of infecting older nymphs (potentially never reaching reproductive maturity), you'd think this would provide an incentive (or to be more technically precise, evolutionary selection pressure) for E. inaron to avoid older whitefly nymphs - but that was not what the researchers found in the study we are featuring today. When they exposed female E. inaron wasps to two different whitefly species - the silverleaf whitefly (Bemisia tabaci) and the banded-winged whitefly (Trialeurodes abutiloneus), they displayed no particular preference for younger or older nymphs.

So why has E. inaron not evolved the ability to distinguish hosts of different ages? After all, other species of parasitoid wasp, such as the aphid parasitoid mentioned above, have evolved the ability to distinguish hosts of different size and shows a preference for hosts of a particular size.

Keep in mind that this tiny wasp is a generalist that infects multiple species of whiteflies -  different species of whiteflies might impose different selection pressures upon the wasp population that prevents them from evolving an optimal approach to selecting the right host. In addition, older whiteflies are likely to be already parasitised by another wasp larva. If a newly arriving larva finds itself in an already occupied host, it can speed up its own development by exploiting the gains of the older, resident larva (a weakened host with an already suppressed immune system). A previous study has shown that when it comes to within-host competition, for E. inaron late-comers often wins.

So instead of being maladaptive, E. inaron that infect older whitefly nymphs may in fact be taking a bit of a gamble - a highly risky one, but one that comes with a potentially high pay-off.

Reference:
Brady, C.M. and White, J.A. (2012) Everyone's a loser: parasitism of late instar whiteflies by Encarsia inaron has negative consequences for both parasitoid and host. Annals of the Entomological Society of America 105:840-845.

Metarhizium acridum

The locust in the photo is covered in a fine layer of green mold - that is because it was killed by the parasite we're featuring today, Metarhizium acridum. Metarhizium acridum is a pathogenic fungus which specifically infects and kills grasshoppers, locust and other insects in the Orthoptera order. Because of the pest status of some orthopterans (think locust plagues), M. acridum is mass-produced as type of environmentally-friendly, biological alternative to most insecticide. But while M. acridum only targets locust and grasshoppers, its close relative, M. robertsii, is far less picky, capable of infecting hundreds of different insect species.

So why is M. acridum so picky while its close relative is so indiscriminate? Amazingly, it appears to come down to a single gene call Mest1 - a gene present in M. robertsii, but is absent in M. acridum. To find out the function of this gene, a group of researchers in China created a mutant M. robertsii strain which has a non-functioning copy of Mest1. This mutant lost its ability to infect most insects - except grasshoppers and locusts - which happens to be the speciality of M. acridium. In parallel, the researchers also inserted functional copies of Mest1 into M. acridum. The insertion of this single gene allowed M. acridium to infect a wider range of insects.

What is so special about Mest1? In M. robertsii, Mest1 is expressed during spore germination, and plays an important role in initiating the infection process. Mest1 expression can be triggered by a range of stimuli including nutrient poor conditions or contact with insect cuticle. Metarhizium acridium has other genes playing the role of Mest1, but they are triggered by substances which are present only in the waxy coating of grasshoppers and locusts. So if its spores land on other insects such as caterpillars which have a different type of coating, M. acridum fails to germinate because the appropriate stimuli are absent. Thus, the insertion of Mest1 into M. acridium allows the fungus to bypass those usual stimuli and begin germinating under a wider range of conditions

Host specificity is one of the central question in the evolutionary biology of parasitic organisms. In this case, we can see how a single gene can changed this otherwise specialist pathogen into a broad-spectrum generalist.

Image from the Wikipedia.

Wang S, Fang W, Wang C, St. Leger RJ (2011) Insertion of an Esterase Gene into a Specific Locust Pathogen (Metarhizium acridum) Enables It to Infect Caterpillars. PLoS Pathog 7(6): e1002097. doi:10.1371/journal.ppat.1002097

November 4 - Camptopteroides verrucosa

Camptopteroides verrucosa is a species of fairyfly (family Mymaridae) - which is to say that it's not a fly at all, but rather a tiny little wasp. The very largest of these wasps only has a wingspan of 3 millimeters, so we are definitely talking tiny! Although not much is known about them, it has been observed that they can move and even mate underwater. The females inject their eggs into those of other insects and use an enormous variety of different hosts. These little parasitoids have recently been used in biocontrol efforts.

September 19 - Phasmarhabditis hermaphrodita

Parasites have to find their hosts and if you're a parasite of gastropods like snails and slugs, that means being attracted to slug slime - i.e. the chemicals found in the mucus secreted by slugs and snails. Phasmarhabditis hermaphrodita is a parasitic nematode that infects and kills its molluscan hosts and has, in fact, been shown to be attracted to their mucus. These parasites are commercially sold as slug pest control agents in the U.K. Other studies have shown that the parasites are somehow able to manipulate the behavior of their hosts to crawl into the soil -this allows the nematodes to complete their development before the dead slug or snail or consumed by a scavenger.

Image is from this site.

January 12 - Wolbachia pipientis

Talk about manipulative little buggers - Wolbachia pipientis are alpha proteobacteria that infect a very wide variety of insects and other arthropods. Like mitochondria, they are vertically inherited from mothers to offspring. But, they get very cranky if they don't get their way. In some cases, they kill off infected males, in some they feminize males, in some they allow females to reproduce parthenogenically, and then there are several documented cases of more complicated interactions in the form of cytoplasmic incompatibilities. Some are now investigating their potential use in biocontrol of malaria vectors and other insect pests, taking advantage of these manipulative tendencies.

Thanks to Mike Charleston for this nomination.

January 5 - Peristenus digoneutis

If you liked the movie Alien, then you might be a fan of parasitic wasps, such as Peristenus digoneutis. These insects, sometimes incredibly tiny, capture and kill other insects (in this case little plant bugs) that they then lay their eggs into. The larvae hatch out and eat their hosts from the inside out. Although it's rather gruesome, these parasitoids have been developed as biocontrol agents to combat insect pests.